1. Field of the Invention
The present invention relates to the developing unit with a magnetic brush used for an electrophotographic copying machine, for example, and more particularly to the structure to mount said developing unit on the machine body.
2. Description of the Prior Art
An ordinary developing unit with a magnetic brush has a structure shown in FIG. 1. And inside the developing box 2 that is fixed to the machine body etc. in the position opposite to the photosensitive substance drum 1, there are provided the developing sleeve 4 having permanent magnets 3 therein, the stirring blades 5 and a doctor blade 6. In the developing unit with a magnetic brush of such a structure, therefore, the ear of the developer regulated by the doctor blade 6 is formed on the circumferential surface 4a of the developing sleeve 4 and electrostatic latent iamges on the circumferential surface 1a of the photosensitive substance drum 1 are developed by toner in the developing area A. Consequently, the minimum distance d between the circumferential surface 1a of the photosensitive substance drum 1 and the circumferential surface 4a of the developing sleeve 4 commands the developing efficiency of the developing unit and it has an important meaning. In the case the developer containing coating carrier whose surface is coated with insulating resin is used, the value of the minimum distance d is 1.0-2.0 mm at most and the dimensional accuracy thereof is a very severe one such as .+-.0.05-0.1 mm when it is intended ot obtain an image with good quality.
Conventionally, by the way, for the purpose of getting accurate value of the minimum distance there have been taken countermeasures such as the method to obtain necessary dimension by inserting spacers to the mounting seat of the machine frame to which the developing box 2 is mounted or the method wherein the driven rollers for positioning are supported by the developing box 2 and these driven rollers are contacted to the circumferential surfaces at the ends of the photosensitive substance drum. However, the former countermeasure has a drawback that the difficult assembly work to insert the thickness guauge etc, in the narrow distance d is required and the reproducibility is detracted when dismounting and mounting the developing unit and consequently the adjustment should be amde again. Compared with this, the latter countermeasure has a merit that the assembly work is easy and the reproducibility is good but it has a drawback that the use of special driven rollers is needed in order to keep the potential between the surface of the photosensitive substance drum and the developing unit and unnecessary length is required to the length of the photosensitive substance drum because the circumferential surfaces at the ends of the photosensitive substance drum will become inefficient and consequently the entire apparatus will become bulky.